To convert (a sound) into a sibilant sound, or to pronounce with a hissing quality.
From ad- + sibilate (from Latin sibilare, 'to hiss' + -ate verb suffix). The Latin sibilare imitates the actual hissing sound of an s, making this word somewhat onomatopoetic in origin.
Linguists use assibilate when a language shifts sounds—like how Latin 'ti' became 'tsi' (the hissing sound) in Romance languages, which is exactly why we say 'assibilate' rather than 'just change.'
Complete word intelligence in one call. Free tier — 50 lookups/day.